There are prior examples where variable capacity diodes were installed in the antenna circuits of portable small model radio instruments, in order to eliminate dispersion of the resonance frequencies of antenna circuits during mass production. However, in these instances, once the resonance frequencies have been corrected with the variable capacity diodes, there is no slippage in the resonance frequencies thereafter, so that the variable capacity diodes are used passively, so to speak.
On the other hand, variable capacity diodes are used with small model radio instruments that are capable of receiving continuously in a certain frequency range, because it is necessary to change the resonance frequency of the antenna circuit continuously. However, they are not used for purposes of correction when there is slippage in the resonance frequency of the antenna circuit for some reason, and there is almost no slippage in the frequency.
Further, the AFC (Auto Frequency Control) circuit system is a system for obtaining stabilized sending and receiving signals, and also uses variable capacity diodes for correction of slippage in signal frequency, but since this is something that corrects the oscillation frequency of the local oscillator in a superheterodyne system, it differs in purport from the present invention.
The present invention applies to an antenna circuit having a loop antenna integrated in a wrist band for wearing on the wrist, and to a wrist radio instrument.
That is, because the loop antenna is worn in a form where it circumscribes the wrist, the length of the loop antenna changes depending on the individual user. Since this means that the inductance value of the loop antenna changes depending on the individual, the resonance frequency of the antenna circuit has changing values depending on the individual, leading to dispersion in the gain of the loop antenna. This has an effect on the sensitivity of the radio instrument of each individual, and when there is severe slippage in the resonance frequency, there is likelihood that sending and receiving signals will become impossible.
On the other hand, after the loop antenna has been placed on the wrist, there is slippage in the resonance frequency from the effect of the human body. This fact means that even if signals can be raised when it is not worn on the wrist, there is a possibility that signals cannot be raised once it is worn.
Since this development arises because of slippage in the resonance frequency of the antenna circuit, we may say that the resonance frequency should be restored to its original form, but having the user of the loop antenna make hand adjustments to do this would be very troublesome and unrealistic.
Here, the object of the present invention lies in offering an antenna circuit capable of correcting slippage in the resonance frequency of the antenna circuit automatically, and maintaining the resonance frequency constantly at a set frequency.
Also, the wrist radio instrument of the present invention, provided with such an antenna circuit, is capable of sending and receiving signals at a constantly set sensitivity, without regard to the wrist thickness of the individual user, and without regard to whether it is being worn on the wrist or not.